I've been having the same issue, "0 assets found!" on all our sites recently. I don't know exactly when it stopped working because we haven't attempted to use Asset Optimization lately, but when Google rolled out its new method of scoring mobile sites we needed to revisit the idea. Now we can't get it to work on any site. I tried disabling ALL plugins other than Hummingbird and switched to the 2017 theme, but still nothing. The debug logs are empty, so there aren't any errors to share. Unfortunately, because of our hosting configuration, I can't give out a cPanel/login info for our devel site, but I reviewed the php info and tested multiple configurations on multiple sites.
Some additional information:
Rackspace server
SSL Installed (and site forced to https)
PHP Version 7.1.23
max_execution_time 30
memory_limit 256M
post_max_size 200M
WordPress 4.9.8
Hummingbird Pro 1.9.2

I've been having this problem as well--all images sizes checked, including original full size image, super-smush enabled, yet Hummingbird and Google still claim images need to be smushed. I checked a specific image that was referenced in Hummingbird and the media library would not let me manually smush it because it had already been smushed. Please post a solution here if you find one.